1. Field of the Invention
The present invention relates generally to adjustable current sources and more specifically to a method and structure for adjusting the offset of differential and operational amplifiers by electrically altering the current source therein.
2. Description of the Prior Art
Operational amplifiers are typically designed with a differential amplifier for the input stage. The balance of the differential amplifier is dependent on component match. Processing variables prevent exact match from occurring. This mismatch appears as offset voltage and/or offset current. In some applications this offset voltage cannot be tolerated and must be adjusted or corrected. There are three basic prior art approaches, i.e.
(1) offset adjustment by external potentiometer;
(2) automatic offset correction by additional circuitry;
(3) offset adjustment by laser trimming of thin film resistors.
The external potentiometer appoach uses a potentiometer external to the IC package to effect offset adjust. This appoach has the disadvantage of requiring an additional component large in comparison to the IC package, thereby requiring extra board space and expense. The "automatic correction circuit" approach requires extensive additional circuitry which increases the chip area and over all cost. The laser trimming approach requires capital equipment investment by the manufacturer. Also, the laser trimming must be performed prior to packaging.
As an advancement over these prior art techniques, U.S. Pat. No. 3,761,787 to Davis et al., described a circuit for internally adjusting the offset voltage of a bipolar transistor operational amplifier using a two segment collector with one of the segments being connected to the collector conductor by fusible metal links. As illustrated in FIG. 2 of that patent, two probe pads 50 and 52 are interconnected by a fusible link 53 and connect the smaller collector 49 to the collector conductor 48. If trimming is necessary, a voltage is applied across the probe pads 50 and 52 to burn out the fusible link 53. This structure requires special probe contact pads and accessible prior to packaging.
The techniques of the Davis et al. provide a rough adjustment of the offset, generally only a simple adjustment. There is a need for the integration of the offset adjustment into the operational amplifier while preserving the fine adjustment of external techniques.